1. Field of the Invention
The invention relates to a device for measuring the pulse transfer spectrum of elastically scattered X-ray quanta, including
a polychromatic X-ray source, PA1 a primary diaphragm device which is arranged between the X-ray source and the examination zone and which serves to form a primary beam irradiating the examination zone as a surface of cone, PA1 a detector device having a plurality of detector elements for detecting X-ray quanta scattered in the examination zone, and PA1 a secondary diaphragm device which is arranged between the examination zone and the detector device and which has imaging slits which enclose, as an are of circle, a system axis extending through the X-ray source.
2. Description of the Related Art
A device of this kind is known from EP-OS 462 658 which corresponds to commonly-owned U.S. Pat. No. 5,231,652 and can be used, for example for the identification, on the basis of their pulse transfer spectrum, of substances present in a piece of luggage, notably crystalline substances (explosives). The secondary diaphragm device thereof is formed by a plate provided with an annular slit. The examination zone is imaged on the detector device through said slit; therefore, such a slit will also be referred to hereinafter as an imaging slit. The likewise annular detector elements are capable of detecting the scattered radiation through the imaging slit, said scattered radiation being generated within the examination zone by the primary beam. The scattered radiation from the part of the examination zone nearest to the system axis is detected by the detector element situated furthest from the system axis, whereas the scattered radiation from the part situated furthest from the system axis is detected by the detector element situated nearest to the system axis. Even though each detector element detects the scattered radiation emanating from the primary beam at a defined scatter angle, the scatter angles associated with the various detector elements deviate from one another.
Because the pulse transfer is proportional to the product of the sine of half the scatter angle and the energy of the elastically scattered X-ray quantum, the ratio of the maximum energy to the minimum energy of the X-ray quanta emitted by the X-ray source must be substantially greater than the ratio of the maximum pulse transfer to the minimum pulse transfer to be measured by each of the detector elements. When the ratio of the maximum pulse transfer to the minimum pulse transfer is, for example 1.8:0.8 (i.e. 2.25:1), a larger ratio of highest to lowest energy of the X-ray quanta used for determining the pulse transfer is required. Depending on the difference in scatter angle, this ratio may be, for example 4:1. This may give rise to problems, because X-ray quanta of low energy are strongly absorbed in the examination zone, whereas X-ray quanta of higher energy are not sufficiently absorbed in customary semiconductor detectors (Ge, Li), so that the sensitivity of these detectors decreases for quanta of higher energy.
In this respect the ratios in the devices disclosed in U.S. Pat. No. 5,007,072 as well as in EP-OS 370 347 are more attractive, because all detector elements detect only the X-ray quanta scattered at the same scatter angle. Therefore, the ratio of the maximum to the minimum quantum energy therein need not be greater than the ratio of the maximum to the minimum pulse transfer. However, a very complex secondary diaphragm device is required to ensure that all detector elements can be struck by scattered radiation only at the relevant scatter angle. The secondary diaphragm device should then comprise a number of elongate collimator bodies which are arranged at a small distance from one another. The manufacture of such a secondary diaphragm device, however, is intricate.